Fuel cells are energy conversion devices that transform the stored chemical energy in fuels such as hydrogen and alcohols and oxidants such as oxygen to electric energy in accordance with electrochemical principles. They have a high energy conversion rate and are environmentally friendly. Since, proton exchange membrane fuel cells (PEMFC) can operate at low temperatures and have high specific power, they are a new power source for both civilian and military applications. For example, they can be used in independent power stations as well as a mobile power source for electric cars and submarines.
Proton exchange membranes are key components in PEMFC. Operating conditions for PEMFC require that the performance and stability of a proton exchange membrane to possess the following properties: first, a high proton conducting capability. As a proton conductor, a proton membrane has to have good proton conducting capabilities, at 0.1 S/cm or higher. In addition, it also has to insulate the electrons. Second, the proton exchange membrane should posses good mechanical properties. The proton membrane has to be strong enough to withstand the fluctuations in gas pressure during the operation of the fuel cell and not break or rupture. Thirdly, the proton exchange membrane should be chemically, hydrolytically, dimensionally and thermally stable.
In the past, the commonly used proton exchange membrane was a Nafion series membrane produced by E.I. DuPont de Nemours and Company in the United States. The Nafion series membrane is a membrane with a perfluorosulfonic structure. It is extremely stable chemically as it has an extremely stable C—F chemical bond. However, these membranes are expensive as they require a special fabrication process. Therefore, it is uneconomical to use these membranes in the commercial production of fuel cells. Furthermore, a large quantity of fluoride is used in the fabrication of the Nafion membranes. As a result, the reaction equipment for the production requires stringent and expensive specifications. In addition, the fabrication process produces harmful environmental pollutants. Therefore, there is a need to find materials for proton exchange membranes that are low in cost and do not contain or only partially contain fluorides.
U.S. Pat. No. 5,795,496 disclosed a method for treating a polymer to improve its proton conducting properties. The method includes the steps of: obtaining a treated polymer material capable of forming a material with requisite proton conducting properties; treating said polymer material using the sulfonation method to increases its proton conduction performance; and cross linking said sulfonic radical to obtain a material with an asymmetric balanced surface density. This method can produce a sulfonated polyether-ether-ketone (S-PEEK) proton exchange membrane. Further heat treating this membrane will produce a proton exchange membrane with a cross-linking structure. Membranes fabricated using this method have higher proton conducting properties and are thermally stable. However, they expand significantly after absorbing water, and have poor dimensional and hydrolytic stability. The structure of a sulfonic radical (—SO3H) that is directly connected to a benzene ring is unstable. When the sulfonic concentration is reduced and the temperature is increased, the sulfonic radical can be easily separated from the benzene ring, resulting in poor hydrolytic properties and limiting the life of the membrane.
EP1296398A2, CA2394674, U.S. Pat. No. 6,670,065B2 and US20030129467 disclosed polymers having an alkyl-sulfonic branch chain and aromatic main chain and their proton exchange membranes that have good hydrolytic resistance stability. U.S. Pat. No. 6,670,065B2 disclosed a solid polymer for electrolytes having a polyether sulfone. This polyether sulfone has a bonded sulfo-alkyl radical with the chemical formula: —(CH2)n-SO3H where n is an integer from 1 to 6. This electrolyte has good hydrolytic resistance stability. However, under dry conditions, the proton membrane made with this polymer is fragile and can easily rupture. Moreover, after absorbing water, the wet proton membrane expands substantially, resulting in a wet membrane with poor dimensional stability and poor mechanical strength.
Due to the limitations of the prior art, it is therefore desirable to have novel polymers for proton exchange membranes that are chemically, hydrolytically, dimensionally and thermally stable.